1. Field of the Invention
This invention pertains to a lighttight cassette with a roll of web material, the cassette housing having a wall surrounding the periphery of the roll and two ends connected through the housing wall, on which ends the roll is supported in a rotatable manner.
2. Description of Related Art
As a rule, the web material is light-sensitive film or paper needed in an exposure device to reproduce originals or to make negatives for the exposure of printing plates. Such cassettes generally have to fulfill two opposing requirements. First, the roll must be supported with the least possible friction to assure easy and smooth transport of the film into the exposure device. This is particularly important in end uses, such as, for example, in certain scanners, where the film is exposed during transport. Each irregularity in transport rate would produce distortions in the exposed image on the film. Secondly, the web material, that is, the film, must not unroll by itself or twist during transport of the cassette to and from the exposure device or while the cassette is stored; the lower the friction of the support bearing, the more easily this occurs. In this event, there is the risk that the film may touch the housing wall and is scratched.
In one known cassette (U.S. Pat. No. 3,612,424), the roll is supported in the cassette on a continuous axis that connects the two end walls together. The axis runs through one end wall and can be provided there with a nut. By the nut being tightened, the end walls are moved towards each other and clamp the roll between them. When the nut is loosened, the end walls move away from each other and the roll is then freely mobile. This design has the disadvantage of relatively high production cost. It can be used only with a continuous axis that connects the two end walls together. Furthermore, tightening the nut produces stresses in the housing, especially the housing wall, which can deform individual housing parts; for example, the stresses can bend the housing wall outwards. Deformation can cause gaps in the contact points or joints between the individual cassette parts, permitting light to enter the interior of the cassette. This is particularly undesirable in cassettes for light-sensitive material.
German OS 21 66 321 discloses a film roll clamped permanently onto both ends so that the film roll can be unwound only against a predetermined resistance, determined by the clamping mechanism. In this instance, as the roll radius decreases, gradually increasing force is required to unroll the film at uniform speed. In each case, sometimes considerable friction must be overcome.
U.S. Pat. No. 4,398,814 discloses a twist preventative in the form of a retention pin that is eliminated when the film is first unrolled. This is only a transport security device that is broken off irreversibly before use. If the cassette has to be made secure for later transport because, for example, another film material has to be used before the entire original content of the cassette is expended, the risk of twist arises again and especially so if the cassette is partially empty.
The object of the present innovation is to provide a lighttight cassette that offers protection from roll twist and that is easy to make and use.